In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
A conventional water-based drilling mud formulation is comprised of basically the following ingredients: water, a clay such as bentonite, lignosulfonate, a weighing agent such as BaSO.sub.4 (Barite), and a caustic material such as sodium hydroxide and a caustic material such as caustic barite, to adjust the pH of the drilling mud to a pH of about 10 to about 10.5.
The continuing search for oil has placed greater demands on the material packages that are utilized to conduct the drilling processes. As drilling depth has increased, so has the bottomhole temperature. In water-based drilling muds, elevated temperatures result in the flocculation of the mud components and thus problematic increases in mud viscosity. Over the years, a progression of different additives has raised the maximum operable bottomhole temperature. Recent disclosures have shown that sulfonated low molecular weight styrene/maleic anhydride copolymer is an effective water-based mud deflocculating additive. This application describes the use of a new sodium styrene sulfonate/sodium-N-(4-sulfophenyl)-maleimide alternating copolymer as an improved additive for high temperature water-based mud deflocculation.
This invention describes an approach to reducing the viscosity of water-based drilling muds which is based on the addition of a water soluble sulfonated copolymer to the mud. The resulting polymer-modified drilling muds display viscosities which are in a desirable range for drilling mud applications after aging at temperatures as high as 400.degree. F. for 24 hours.
The types of sulfonated polymers that are envisioned in the present invention are sodium styrene sulfonate-co-sodium-N-(4-sulfophenyl)-maleimide copolymers.